ABSTRACT LXXIII, August 2004 n. 1:
Massimo
Mattei, Nicola D’Agostino, Claudio
Faccenna, Claudia Piromallo and Federico Rossetti - Some remarks on the geodynamics of the Italian
region
Abstract - In this paper we present geological and geophysical data which constrain
the Tertiary and Quaternary evolution of the Italian region, relevant to the
interpretation of the genesis of magmatism in the frame of the geodynamic
processes. GPS results show that, at the longitude of Sicily, the African
approaches the Eurasian plate at a velocity of about 5 mm/yr in a NW direction.
Furthermore, data show that the Adriatic foreland presently moves independently
from the African plate. Geodetic and seismic data show that NW-SE oriented
extension is the main active tectonic process in the Apennine chain.
Relative and absolute motions of the Africa and the
Eurasia plates indicate several hundred kilometers of convergence since the
early Tertiary, in the central Mediterranean. This convergence has been
achieved by northwestward dipping subduction of the African plate. The main
present-day geodynamic feature of the Italian region is represented by
seismicity on a well defined Benioff zone, which reveals a still active process
of subduction from the Ionian foreland below the Calabrian Arc and Tyrrhenian
Sea. This slab is the result of a NW directed long running subduction process
active since the Tertiary, which consumed the Ligure oceanic basin first, then
a small fragment of the Apulian continental lithosphere, and finally most of
the present-day Ionian lithosphere, whose subduction is still ongoing.
We also suggest that the lateral break-off of the
Ionian subducting lithosphere could allow lateral astenospheric flow above the
subducted plate either from the Apulian plate and from the Sicily
Channel-western Sicily.
Pietro Armienti, Sonia
Tonarini, Massimo D’Orazio and Fabrizio Innocenti - Genesis and evolution of Mt. Etna alkaline
lavas: petrological and Sr-Nd-B isotope constraints
Abstract - Mt. Etna lies at the northern margin of the African plate, on the accretionary
prism of the Africa-Europe subduction system. Differential roll-back of the
Ionian oceanic lithosphere has created a vertical slab window through which
the passive rise of asthenosphere causes partial melting and magma genesis.
In this rapidly evolving geodynamic context, the increase of alkalinity in
time, accompanied by variations of Sr and Nd isotopes, is a first order feature.
Fluid mobile elements and B systematics reveal the fundamental role played
by fluids released by the dehydrating oceanic lithosphere, even if they are
added to the mantle source in amounts no larger than 1 wt.%. This induces
relatively high H2O contents in Etna magmas that exert a strong
control on phase relationships. Compositions of alkaline primary melts have
been reconstructed and used to constrain the depth of origin and fractionation
sequences at various pressures. It was found that phase relations at the crust-mantle
boundary can only produce basaltic compositions akin to the less evolved sub-aphyric
lavas, whereas trachybasaltic compositions are generated in the depth range
12-3 km together with cumulate bodies detected through geophysical investigations
and here related to massive pyroxene and plagioclase fractionation. Detailed
monitoring of Sr-isotope equilibrium between pyroxene and host rock revealed
both the increase of 87Sr/86Sr over time and the occurrence
of mixing processes between distinct batches of magma in the plumbing system.
Sr, Nd and B systematics also reveal contamination effects due to interactions
with lower crust and sporadic interaction with the sedimentary basement.
Teresa Trua, Giancarlo
Serri, Michael P. Marani, Piermaria L. Rossi, Fabiano Gamberi
and Alberto Renzulli - Mantle domains beneath the southern Tyrrhenian: constraints from recent
seafloor sampling and dynamic implications
Abstract - Southern Tyrrhenian Quaternary magmatism represents one example of an
active arc/back-arc system where IAB- and OIB-type magmas coexist. IAB-type
magmatism is the most common, in both arc and back-arc settings, whereas OIB-type
magmas are restricted to few areas. Geochemical and isotopic characteristics
of southern Tyrrhenian submarine volcanic samples are here summarized, with
special attention to those samples recovered during recent seamounts/seafloor
sampling. Petrological data of the most basic lavas have been evaluated with
the aim to characterize the mantle sources of IAB and OIB magmas. These data
provide important insights into the petrogenesis of the southern Tyrrhenian
submarine magmatism and on the possible geodynamic scenario able to explain
the coexistence of IAB and OIB magmas in this area.
Lorella Francalanci, Riccardo Avanzinelli, Chiara
M. Petrone and Alba P. Santo - Petrochemical
and magmatological characteristics of the Aeolian Arc volcanoes, southern
Tyrrhenian Sea, Italy: inferences on shallow level processes and magma source
variations
Abstract - The Aeolian volcanic arc, constituted by seven islands and several seamounts,
is emplaced on continental lithosphere. The islands are mainly formed by lava
flows, domes and pyroclastic deposits, and emerged from the sea in a short
time span, from around 200 ka ago at Filicudi, Lipari and Stromboli (Strombolicchio
neck), to about 100 ka at Alicudi and Stromboli. At Panarea, an intense fumarolic
activity is still present, the last eruptions at Lipari took place on 580
A.D., whereas Vulcano and Stromboli are still active. The rock compositions
belong to different magmatic series and show a large silica range (48-76 wt%).
Calc-alkaline (CA) and high-K calc-alkaline (HKCA) volcanics are present
in all the islands, except for CA rocks at Vulcano. Shoshonitic (SHO) products
are only lacking at Alicudi, Filicudi and Salina. Potassic (KS) volcanics
have been erupted at Vulcano and Stromboli. Basalts are not found at Lipari,
whereas a large amount of rhyolites are present in the central arc islands
(Lipari, Vulcano, Salina, Panarea), having different petrochemical characteristics.
87Sr/86Sr increases from the western to the eastern
sectors of the arc (0.70342-0.70757), whereas 143Nd/144Nd
decreases (0.51289-0.51243). Pb isotope ratios show a large similar range
in the western and central arc islands, but decrease at Panarea and Stromboli
(e.g., 206Pb/204Pb: 18.93-19.77). Among CA and HKCA
rocks, incompatible trace element contents and ratios change passing from
the central part of the arc to the external sectors. The isotopic and geochemical
compositions of SHO and KS volcanics from Stromboli and Vulcano are distinct,
with the Vulcano compositional characteristics resembling those of the CA
and HKCA magmas from the central arc. Significant rock compositional variations
are also observed within the single volcanoes.
Aeolian
magmas underwent multiple differentiation processes during the ascent to the surface
from their mantle source. Fractional crystallisation is often associated to
crustal contamination which affected at higher extent either the most evolved
magmas at Vulcano, Salina and Lipari or the most mafic magmas at Alicudi,
Filicudi and Stromboli. Multiple mixing also played an important role often
associated with the other differentiation processes. These evolutionary
processes occurred at polybaric conditions, with higher crystallisation
pressure for Filicudi and Salina.
The
different parental magmas were originated in an heterogeneous mantle wedge,
metasomatised by subduction-related components at increasing extent, going from
west to east (= variation of Sr and Nd isotope ratios). It is also suggested
that the mantle source of CA and HKCA magmas from central arc (Salina, Lipari
and CA Panarea) is a MORB-like asthenospheric source, contaminated by aqueous
fluids released by subducted oceanic crust + pelagic sediments. The same type
of source can be envisaged for Vulcano SHO and KS parental magmas. In the
external sectors of the arc, mass transfer from the subducted slab to mantle
seems to be occurred by higher melt/fluid proportions. At west (Alicudi and
Filicudi), the other components involved in the magma genesis remain similar to
those proposed for the central arc source. On the contrary, the compositional
characteristics of eastern magmas (HKCA Panarea and Stromboli) suggest a
different pre-contamination mantle source (continental lithosphere?) and/or a
different crustal contaminant (with low Pb isotopes) of the mantle wedge. A
decreasing partial melting degree of distinct mantle sources is considered to
generate magmas with an increasing potassic character at Vulcano and Stromboli.
Michele Lustrino, Vincenzo Morra, Leone Melluso,
Pietro Brotzu, Fosco d’Amelio, Lorenzo Fedele, Luigi Franciosi, Roberto Lonis
and Alfredo Massimo Petteruti Liebercknecht - The Cenozoic igneous activity
of Sardinia
Abstract - During the Cenozoic, the island of Sardinia was the location of two different
magmatic episodes: 1) a Oligocene to Miocene (hereafter OM) cycle ~32-15 Ma
and 2) a Pliocene to Quaternary (hereafter PQ) cycle (~5-0.1 Ma). These two
volcanic cycles differ in many aspects: 1) geographic occurrence [the OM rocks
occur almost exclusively in a graben structure called the Fossa Sarda (Sardinian Trough) that cuts
the entire island from north to south, whereas the PQ rocks are scattered
throughout the island]; 2) petrography (the OM rocks are mostly porphyritic,
whereas the PQ rocks are mostly aphyric); 3) geochemical affinity (the OM
rocks are mostly subalkaline with a tholeiitic to calcalkaline character,
whereas the PQ rocks are mostly sodic alkaline with fewer tholeiitic types);
4) major element compositions [the OM rocks are mostly dacites to rhyolites
with fewer basaltic andesites, andesites and rare basalts while the PQ rocks
are mostly hawaiites, mugearites and basaltic andesites, with both SiO2-oversaturated
and SiO2-undersaturated evolved types (rhyolites and phonolites);
moreover, for a given SiO2, OM rocks have higher CaO, lower TiO2
and lower Na2O compared to the PQ rocks]; 5) trace element abundances
and ratios (the OM rocks have lower HFSE and REE contents and higher La/Nb
and Zr/Nb ratios compared to the PQ samples); 6) Sr isotopic composition (the
OM rocks have 87Sr/86Sr generally > 0.7047, whereas
the PQ rocks have 87Sr/86Sr generally < 0.7050).
143Nd/144Nd and 206Pb/204Pb ratios
of the OM rocks (from 0.5127 to 0.5122 and from 18.52 to 18.71, respectively)
fall within the range of the PQ samples (from 0.5129 to 0.5122 and from ~17.5
to 19.42, respectively).
The
OM rocks show geochemical features typical of magmas emplaced in
subduction-related settings. They are believed to have been generated within
the mantle wedge developed above a west-directed subduction of (Mesogean?)
oceanic lithosphere below the southern continental margin of Europe. On the
other hand, the PQ volcanic rocks were emplaced concurrent with the formation
of the Tyrrhenian Sea and share some geochemical similarities with magmas
emplaced in within-plate (anorogenic) tectonic settings, although they exhibit
peculiar characteristics.
The
PQ rocks can be divided into two groups: one group (Unradiogenic Pb Volcanics =
UPV) has relatively high 87Sr/86Sr (0.7043-0.7051), low 143Nd/144Nd
(0.5124-0.5126), and is characterised by the least radiogenic Pb isotopic
composition so far recorded in Italian (and Circum-Mediterranean) Cenozoic
igneous rocks (206Pb/204Pb = 17.36-18.07); these are the
most widespread volcanic rocks and crop out in central and northern Sardinia.
The other group (Radiogenic Pb Volcanics = RPV) has chemical and Sr-Nd-Pb
isotopic ratios indicative of a markedly different source (87Sr/86Sr
= 0.7031-0.7040; 143Nd/144Nd = 0.5127-0.5129; 206Pb/204Pb
= 18.8-19.4), and crop out only in the southern part of the island. The less
differentiated rocks of the two groups also show distinct trace element
contents and ratios (e.g. Ba/Nb>14, Nb/U <38 and Ce/Pb <20 for the
UPV; Ba/Nb<9, Nb/U >45 and Ce/Pb >24 for the RPV).
The transition from igneous cycles with orogenic
(s.l.) to anorogenic (s.l.) geochemical features is relatively common
throughout the entire circum-Mediterranean area; in other places this shift of
chemical compositions has been related to «slab detachment» and/or the
development of «slab window» processes. In this paper, to explain the
geochemical differences between the OM and PQ volcanic products of Sardinia, we
propose the involvement of different mantle sources: an asthenospheric mantle
source slightly modified by subduction-related metasomatism for the OM rocks
and a lithospheric mantle source strongly modified during ancient times
(possibly during Hercynian orogenesis) for the great majority of the PQ
volcanic rocks.
Sandro Conticelli, Leone Melluso, Giulia
Perini, Riccardo Avanzinelli and Elena Boari - Petrologic, geochemical and isotopic characteristics of potassic and ultrapotassic
magmatism in central-southern Italy: inferences on its genesis and on the
nature of mantle sources
Abstract - Miocene to Quaternary magmatic rocks, found along the Tyrrhenian border
of peninsular Italy, mostly belong to potassic and ultrapotassic suites. They
can be divided into three different petrographic provinces, where magmatism
is confined in terms of space, time and petrographic characteristics.
The Tuscan Magmatic Province
is the northernmost province, in which mantle-derived potassic and
ultrapotassic, leucite-free volcanic rocks occur, prevailing over high
potassium calc-alkaline rocks, and covering a time span of activity between
14.2 and 0.19 Ma. These rocks are silica-saturated to silica-oversaturated and
range from high-potassium calc-alkaline to ultrapotassic lamproite, through
potassic and ultrapotassic shoshonitic series.
The Roman Magmatic Province
extends from Northern Latium to the Umbrian and Campanian areas, arranged in a
volcanic belt along the Tyrrhenian border of the Apennine chain. It is made up
of rare shoshonitic rocks (KS) and leucite-bearing rocks (HKS). Some HKS may
contain melilite, and therefore belong to the kamafugitic group (KAM). Minor
high potassium calc-alkaline rocks are also found in the Pontine archipelago
and in drill in the Campanian plain. Volcanism has been active from 0.7 Ma to
0.1 Ma in the northern districts of the Latian area (i.e., Vulsinian, Vico,
Sabatinian, Alban, Hernican, Auruncan) whereas in the southernmost portion of
the province, the Neapolitan district, shoshonitic and ultrapotassic magmatism
are consistently younger than the Latian ones, ranging from 0.3 Ma to present. Historical
eruptions in the Neapolitan district are indeed recorded at Phlegrean Fields,
Procida, Ischia and Vesuvius volcanoes.
The Lucanian Magmatic Province
is the easternmost volcanic region characterized by rocks rich in both Na and
K. Most of the rocks are haüyne- and leucite-bearing, and were erupted at Monte
Vulture volcano between 0.6 and 0.1 Ma. Carbonatites have been described in the
last phase of activity.
Minor amounts of K-rich rocks are also found in the Aeolian Arc, in the southern Tyrrhenian Sea. These rocks are
intimately associated with calc-alkaline rocks at Vulcano, Vulcanello, and
Stromboli.
Enrichment in K2O and related incompatible trace elements is
accompanied by strong to mildly fractionation of high field strength elements
with respect to large ion lithophile elements. This can be attributed to the
input of a crustal component into the mantle source of the magma prior to
partial melting. Variations in trace element enrichment and isotope
characteristics of the three magmatic provinces are thought to be the result of
different metasomatic events and complex processes of partial melting of the
mantle sources. Peculiar geochemical and isotopic characteristics of the
Lucanian and Neapolitan regions are the result of different channelling of
withinplate material through lateral inflow from foreland, during the roll-back
of the Ionian subduction. Metasomatism affected lithospheric mantle sources
characterised by variable degrees of depletion.
The peculiar petrologic, geochemical and isotopic features of the mafic
magmas are consistent with a post-orogenic subduction-related geodynamic
setting for the production of their parental magmas.
Angelo Peccerillo - Carbonate-rich pyroclastic
rocks from central Apennines: carbonatites or carbonated rocks? A commentary
Abstract - The Pleistocene association of melilititic ultrapotassic kamafugites and
carbonate-rich pyroclastic rocks from Apennine has been interpreted to represent
a melilitite-carbonatite province, such as those typically found in intra-continental
rift settings. Kamafugitic lavas have incompatible element and REE patterns
that closely match those of leucite tephrites from the nearby Roman Province.
Carbonate-rich pyroclasts have similar trace element patterns as kamafugites
and Roman rocks, but, when single carbonate-rich pyroclastic rocks are compared
with the associated kamafugitic lavas from the same volcano, they show a depletion for all the incompatible elements,
which becomes stronger as the amount of carbonate fraction increases. This
shows that the carbonate fraction is geochemically barren and its presence
in the carbonate-rich rocks generates a dilution for almost all the elements.
Oxygen isotope ratios of carbonates in these volcanic rocks are invariably
high (d18O in the range +21 to +
28). Phenocrysts from kamafugitic lavas also have high concentrations of heavy
oxygen (d18O = +11 to + 14).
These data cast serious doubts on the hypothesis that the carbonate-rich
pyroclastics from central Italy represent carbonatitic magmas. Geochemical and
isotopic data strongly suggest that these rocks represent fragmented
kamafugitic magmas, which have suffered addition of geochemically barren
carbonate material from wall rocks. Geological data make this hypothesis
likely, inasmuch as the volcanoes from the internal Apennines are monogenetic
centres, which cut through some thousand meters of carbonate rocks of the
Apennine sedimentary sequences.
Gianluca Bianchini, Luigi Beccaluva and Franca Siena - A reappraisal of ultra-alkaline Intra-Apennine volcanism in central-southern
Italy: evidence for subduction-modified mantle sources
Abstract - The ultra-alkaline Intra-Apennine Volcanism (IAV), which includes kamafugites
(central Italy) and melilitites (Vulture) generally associated with carbonatitic
rocks, is reviewed in order to assess its tectono-magmatic significance. Primitive
mantle normalised incompatible element distributions and Sr-Nd isotope systematics
of IAV products are completely different from the Cenozoic anorogenic magmas
of the Mediterranean area. Similar geochemical differences are observed between
mantle xenoliths associated to anorogenic magmas and those from IAV, which
are characterised by the widespread presence of phlogopite. Moreover, the
IAV ultra-alkaline magmas (including carbonatites) totally differ, in terms
of Sr-Nd isotopes, from those of typical continental intra-plate rift systems,
such as the western branch of the African rift and the northern border of
the Paranà basin. By contrast, the high Sr and low Nd isotopic ratios recorded
in the IAV kamafugites of central Italy, are strictly comparable to those
observed in potassic volcanics of the Roman Magmatic Province (RMP). This
implies a common generation of IAV and RMP magmas from subduction-modified
mantle sources with the possible recycling of continental crust material during
the Apenninic orogenic events, as already suggested by several authors. The
location of the IAV products above the verticalised relict-subducted Adriatic
lithosphere suggests that it is affected by important tectonic discontinuities,
or real slab break-off, which allowed access of subduction components to the
IAV mantle sources. Extremely low partial melting degrees of highly metasomatized
and hybridised mantle sources, deep in the lithosphere, could account for
the generation of IAV ultra-alkaline/carbonated melts.
Giampiero Poli -
Genesis
and evolution of Miocene-Quaternary intermediate-acid rocks from the Tuscan
Magmatic Province
Abstract - Occurrence of a large variety of rock types, intrusive and effusive, closely
associated in space and time reveal a complex magmatic setting for the Tuscan
Magmatic Province. Extensive petrological and geochemical investigations carried
out over the later years indicate that main rock associations are represented
by three groups of rocks at different degrees of evolution: i) mafic ultrapotassic
rocks with lamproitic affinity, and high-potassium calc-alkaline and shoshonitic
rocks; ii) intermediate-acid rocks bearing strong petrographic and geochemical
evidence of magma interaction processes; iii) acid volcanics and intrusives
showing petrological and geochemical characteristics of both extremely evolved
and pure anatectic melts. Literature and new data suggest that a process of
interaction between basic and acid end-members is responsible for the evolution
of the Tuscan Magmatic Province magmatism. Major and trace elements and isotopic
systematic help to recognize the basic end-members as compositionally akin
to three basic-intermediate magmas belonging to Capraia shoshonites, Capraia
high potassium calc-alkaline rocks rich in Sr, and lamproites from Tuscan
area that acted together even in a single intrusive or effusive complex. The
acid end-members in the interaction process are crustal anatectic melts derived
by partial melting at ca 4-6 kbar of gneiss and garnet micaschists of the
Tuscany basement having a sedimentary protolith. Residual assemblages of the
partial melting process calculated by geochemical models agree with experimental
petrological data, and help to reconstruct levels of melting and emplacement
for intrusive complexes, and level of crystallization of phenocrysts for the
effusive ones. The petrological model reported in this work fits well with
geophysical data indicating a superposition of upper crust of both the European
and Adriatic plates in westernmost Tuscany.
Elisabetta Rampone
- Mantle dynamics during
Permo-Mesozoic extension of the Europe-Adria lithosphere: insights from the
Ligurian ophiolites
Abstract - Petrologic and isotope investigations on the Ligurian ophiolites have
provided evidence that they do not resemble oceanic lithosphere formed at
a mid-ocean ridge setting, rather, they represent peculiar and atypical sectors
of oceanic lithosphere composed by older lithospheric mantle peridotites (Proterozoic
and Permian) intruded by younger MORB-type magmas (Jurassic and late-Jurassic).
The Ligurian ophiolites thus reflect a lithologic association which develops
in response to passive lithosphere extension and slow-spreading oceanization,
and which characterizes embryonic stages of evolution of an oceanic basin.
Mantle peridotites from the External Liguride (EL, Northern Apennines) and
the Erro-Tobbio (ET, Ligurian Alps) ophiolitic units both record a tectono-metamorphic
subsolidus evolution characterized by progressive recrystallization from spinel-
to plagioclase- to amphibole-bearing assemblages and deformation from granular
to tectonite- to mylonite- types. Sr, Nd and Os isotope investigations have
indicated that the EL lherzolites were accreted to the subcontinental lithospheric
mantle since Proterozoic times. Sm-Nd dating on the plagioclase-facies recrystallization
stage have yielded 273-313 Ma in the ET peridotites and 165 Ma in the EL lherzolites.
The ET and EL peridotites thus represent different pieces of subcontinental
lithospheric mantle which experienced tectonic exhumation during distinct
stages of extension of the Europe-Adria continental lithosphere, leading to
the formation of the Jurassic Ligurian Tethys ocean. Results on the ET peridotites
point that the decompressional evolution of lithospheric mantle was already
active since Late-Carboniferous-Permian times. A striking feature of oceanic
basins developed by passive lithosphere extension is therefore the tectonic
sea-floor exposure of large sectors of subcontinental lithospheric mantle.
This is consistent with the results of petrologic and structural investigations
on mantle peridotites from modern oceanic analogues (embryonic oceans and
passive continental margins). Another peculiar feature of the Ligurian ophiolites
is the predominant lack of a mantle-crust cogenetic link, as it would be expected
in mid-ocean ridge type oceanic lithosphere. Both the EL and ET peridotites
represent subcontinental lithospheric mantle whose tectonic exhumation was
even completely unrelated to mantle melting and melt production. Sm/Nd isotope
studies on the depleted mantle peridotites from the Internal Liguride (Northern
Apennine) ophiolitic units provided Permian (275 Ma) DM model age of depletion.
Associated gabbroic rocks have been dated to 164 ± 14 Ma. Thus, even in the
IL ophiolitic sequences, residual mantle and associated crustal rocks are
not cogenetic and coheval. Sm/Nd isotope data on the depleted ophiolitic peridotites
from Mt. Maggiore (Corsica) have furnished Jurassic (165 Ma) DM model age
of depletion. Associated gabbroic rocks have been dated to 162 + 10
Ma. The Mt. Maggiore gabbro-peridotite association thus constitutes the first
record of the attainment of a mature oceanic stage of the Ligurian Tethys
ocean, where residual peridotites and associated magmatic rocks are in isotopic
equilibrium.
Angelo Peccerillo
and Eugenio Turco
- Petrological and geochemical variations of
Plio-Quaternary volcanism in the Tyrrhenian Sea area: regional distribution
of magma types, petrogenesis and geodynamic implications
Abstract - The Plio-Quaternary magmatism in the Tyrrhenian Sea area shows a strong
regionality in the distribution of magma types, which allows distinguishing
several magmatic provinces or zones. These are: 1) Tuscany, 2) Latium, 3)
Intra-Apennine region, 4) Neapolitan area, 5) Ernici-Roccamonfina, 6) Mt.
Vulture, 7) Aeolian Arc, 8) Sicily, 9) Tyrrhenian Sea floor, and 10) Sardinia.
The different provinces are sometimes divided by important tectonic lines
and show distinct mechanical characteristics of the lithosphere (i.e., Moho
depth, lid thickness, occurrence of deep seismicity, etc.).
In Tuscany, mafic calc-alkaline to ultrapotassic magmas coexist with
silicic intrusive and extrusive rock of crustal anatectic origin. Magmatism in
central Italy ranges from calc-alkaline to ultrapotassic, but each province
displays peculiar major and/or trace element and/or isotopic composition.
Calc-alkaline to shoshonitic magmatism dominates in the Aeolian Arc. Igneous
activity in the Sicily and Sardinia provinces consist of tholeiitic to
Na-alkaline products, which show low LILE/HFSE ratios, typical of intraplate
volcanics. Intraplate and arc-type rocks coexist on the Tyrrhenian Sea floor.
Radiogenic isotope compositions (Sr, Nd, Pb) of mafic rocks show
moderate within-province variations. However, when the whole Plio-Quaternary
magmatism in considered, continuous trends are observed, which connect various
mantle compositions. Rocks from the Sicily province have HIMU-type signatures
composition trending toward DMM-EM1. Rocks from Sardinia and the Tyrrhenian Sea
plot between EM1 and HIMU composition. Rocks from the Aeolian Arc to Tuscany
define a trend connecting the HIMU-like rocks of Sicily with a Tuscany mafic
ultrapotassic end-member characterised by high 87Sr/86Sr,
low 143Nd/144Nd and moderately low 206Pb/204Pb.
This end-member is more enriched in radiogenic Sr than EM2 mantle component,
and resembles closely the upper crust for both trace element ratios and
radiogenic isotope ratios.
The anorogenic trace element signatures of magmatism in Sicily and
Sardinia suggest a genesis in a mantle which suffered little or no
contamination by subduction processes, at least in recent times. Radiogenic
isotopic trends reveal an interaction between various mantle components (HIMU,
DMM, EM1). In contrast, the magmatism in the Aeolian Arc and peninsular Italy
highlights interaction between HIMU-type and upper crustal reservoirs. Geochemical
modelling demonstrates that such an interaction cannot have occurred during
magma ascent to the surface (magma contamination), but took place in the upper
mantle by addition of upper crustal material (source contamination). Such a
conclusion implies a subduction-related origin for the magmatism in the Aeolian
Arc and the Italian peninsula. The coexistence of anorogenic-type and arc-type
rocks in the Tyrrhenian Sea calls for a role of both intraplate and
subduction-modified mantle sources.
The hypothesis that best explains the regionality of magma types in
Italy and their age, geochemical and isotopic characteristics is that
subduction of Ionian-Adria plate affected the Sardinia block during
Oligo-Miocene and successively shifted south-eastward towards its present
position in the southern Tyrrhenian Sea. This caused a migration of
subduction-related magmatism from Sardinia, through the Tyrrhenian Sea to
southern Italy and the Aeolian Arc. Contemporaneously, the back-arc opening of
the Tyrrhenian Sea basin generated passive asthenospheric upwelling and the
formation of anorogenic-type magmas, which coexist with subduction-related
rocks. The Apennine chain underwent anticlockwise rotation and segmentation
during Tyrrhenian Sea opening, breaking into various arc sectors along which
variable types of sedimentary material were subducted. These induced strong
heterogeneities in the mantle wedge, which are reflected by regional variations
of geochemical and isotopic composition of the magmatism along the Italian
peninsula. The hypothesis of the presence of deep mantle plumes in the
Tyrrhenian Sea area is unable to explain most of the compositional
characteristics of the magmatism. However, the HIMU-like isotopic signatures of
rocks in Sicily (e.g. Etna, Ustica, Iblei) resemble closely those of several
intraplate volcanics in central and eastern Europe (e.g. Massif Central in
France, Eifel region in Germany). This calls for a wide European mantle
reservoir, may be well explained by a deep mantle plume, but does not
necessarily require one.